Filter device

ABSTRACT

A filter device, comprising at least one first connection (A 1 ) to a hydraulic working Circuit, with which a filter element (F 1 ) is associated, and comprising at least one additional connection (A 2 ), by means of which leakage oil amounts of hydraulic components can be fed, is characterised in that a control apparatus (V 2 ) is provided, by means of which the leakage oil amounts can be fed from the additional connection (A 2 ) to the filter element (F 1 ) when a specified working pressure of the working Circuit is not reached at the first connection (A 1 ).

The invention relates to a filter device with at least one first port in a hydraulic working circuit, to which a filter element is assigned, and with at least one additional port, which can be used to supply the leakage oil of hydraulic components.

Filter devices of this type are prior art in a wide range of designs. In hydraulic systems, in which the return flow from working hydraulics, for example in the form of working cylinders, is filtered in an open hydraulic circuit, wherein from a closed hydraulic circuit of the associated hydrostatic drive, such as pumps or motors, oil leakage flows incur, problems arise in connection with the filtration the resulting leak-oil. In view of maintaining the required system purity, the filter element of the working filter, filtering the return flow from the working hydraulics, has to be designed with a high grade of filtration to ensure microfiltration of the return flow. If the return flow is high volume, if the hydraulic fluid is high viscosity (cold conditions) or in case of considerable contamination of the filter element, relatively high back pressures result at the working filter. This precludes using the working filter designed for large filtration grade for superfine filtration of resulting leak-oil, as the leak-oil producing components, such as pumps or motors, do not tolerate higher leak-oil pressures because of their shaft bushings and gaskets. It is therefore necessary to make provisions for leak-oil filtration, for example, separate leak-oil filters of a coarser filtration grade, to avoid higher back pressures, which in turn has negative effects on system purity.

With regard to this problem, the object of the invention is to provide a filter device of the type under consideration, allowing for an efficient filtration of any incurred leak-oil flow in a particularly simple manner.

According to the present invention, this object is achieved by a filter device having the features of claim 1 in its entirety.

According to the characterizing portion of claim 1, one special feature of the invention is that a control device is provided, by which means, if the working pressure of the working circuit at the first port falls below a specified value, the leakage oil quantities can be supplied to the filter element by an additional port. Thereby the invention makes use of the fact that in recent times, in particular for mobile devices in an open hydraulic working circuit, more and more fixed displacement pumps are replaced by variable pumps in the interest of improved energy efficiency. As a consequence, the quantities of oil that are returned through the working filter having a filter element designed for high grade filtration vary greatly in accordance with the load profile of the relevant working hydraulics and decrease in absolute terms. This means that during prolonged periods of operation, in which the variable displacement pump in question performs at partial load or is completely swiveled back, there is no or very little back pressure at the filter element of the working filter. The control device provided in accordance with the invention routes the leak-oil flow from the additional port to the filter element, to utilize the working filter for the ultra-fine filtration of the leak-oil flow during these operating phases, i.e. whenever the working pressure of the working circuit falls below a pre-selectable value at the first port. By using the microfilter of the hydraulic working circuit for filtering the leak-oil flow, a better overall oil quality can be achieved.

In an especially advantageous exemplary embodiment, the control device has a valve assembly having at least one check valve, which connects the additional port to the first port, if the leak-oil pressure at the additional port exceeds the working pressure at the first port. This provision ensures in a simple way that any pressure building up at the microfilter cannot act on the leak-oil port.

An arrangement in which the control device has a second check valve connected to the additional port, i.e. the leakage oil port, is of particular advantage, which valve opens when a desired range of the leak-oil pressure is exceeded and connects the additional port to a branch line leading to a return or tank side. This way, the control device effectively limits the pressure of the leakage oil.

In particularly advantageous exemplary embodiments, a second filter element is provided in the branch line leading from the second check valve to the return or tank side, having a filter grade which is coarser than that of the first filter element disposed between the first port and the return or tank side. This way, filtration of the leak-oil also occurs in those operating phases in which the leak-oil port is disconnected from the port at the fine filtering working filter, wherein a detrimental increase in the leakage oil pressure is avoided by a coarser filter grade of this effective second filter element.

Particularly advantageously, the control unit may have a third check valve, connecting the first port to the additional port, as soon as a pre-determined maximum value of the working pressure at the first port is exceeded, thereby bypassing the first filter element via a second filter element.

Particularly advantageously, a further bypass, bypassing the second filter element can also be set up, by providing a fourth check valve, which is opened by the pressure present at the second port. A screen can be placed in the other bypass as a so-called “last chance filter”.

In advantageous exemplary embodiments, a supply port may be connected to the clean side of at least the first and/or second filter element for feeding filtered hydraulic fluid to hydrostatic components such as feed pumps.

To safeguard the supply of such components via the supply port a configuration in which the clean side of the first and/or second filter element, and thus the supply port, is connected to a tank via a replenishment means, which has a fifth non-return valve in the form of a suction valve and a sixth non-return valve for a desired loading of the hydraulic fluid present at the supply port, is particularly advantageous.

A high purity of the system is ensured thereby, if the replenishment means has a suction filter, filtering the corresponding replenishment volumes, in series with the fifth check valve.

In a particularly advantageous manner, the filter device may have a filter housing defining a longitudinal axis, which can accommodate, in a sequential configuration and interconnected interrelated, a first filter element serving as a microfilter and a second filter element in the form of a coarse protective filter. By arranging both filters in a common housing, not only can a compact design be realized, but the device is also easy to maintain, because only one component needs to be replaced in order to change the filter.

Further advantageous embodiments of the filter device are indicated in the relevant dependent claims 12 to 25.

In accordance with claim 26, a filter element, which is provided for use in a filter device according to one of the claims 1 to 25, is also the subject matter of the invention.

The invention is explained in detail, with reference to the drawings, below. In the drawings:

FIG. 1 is a circuit diagram of the hydraulic components of a first exemplary embodiment of the filter device according to the present invention;

FIG. 2-5 show corresponding diagrams of further exemplary embodiments of the invention;

FIG. 6 is a front view of the filter housing of an exemplary embodiment of the filter device according to the present invention;

FIG. 7 is a longitudinal section of the filter housing of the exemplary embodiment of FIG. 6;

FIG. 8 is an enlarged (in comparison to FIG. 7), oblique perspective view of the filtration insert made up of a combination of two filter elements of the exemplary embodiment of FIG. 6 and FIG. 7;

FIG. 9 is an enlarged—in comparison to FIG. 6 and FIG. 7—and longitudinally sectioned perspective oblique view of only the part of the exemplary embodiment of FIG. 6 and FIG. 7 that is adjacent to a removable head piece;

FIGS. 10 and 11 in the depiction, corresponding to FIG. 9, show the central housing part and/or the part of the exemplary embodiment of FIG. 6 and FIG. 7 that is adjacent to a bottom part of the housing;

FIG. 12 is a front view of the filter housing of another exemplary embodiment of the filter device according to the present invention corresponding to FIG. 6;

FIG. 13 is a longitudinal section of the filter housing of the exemplary embodiment of FIG. 12, corresponding to FIG. 6;

FIG. 14 is a depiction, corresponding to FIG. 9, of only the section of the exemplary embodiment adjacent to the head piece in accordance with FIG. 13 and

FIG. 15 shows in a representation corresponding to FIG. 11 only the part of the exemplary embodiment of FIG. 13 and FIG. 14 that is adjacent to a bottom part of the housing.

The circuit example in FIG. 1 shows a port to which a fluid return flow is directed from a working hydraulics, not shown, referred to as A1. For microfiltering the return flow, the port A1 is connected to a filter element F1 with a filter grade in the range 5-10 μm, the clean side 1 of which leads to a tank side 3. Leak-oil, incurred in the operation of hydrostatic drives (not shown) for the working hydraulics concerned, is supplied via a second port A2. This leak-oil line 5 connected to this drain oil port A2 is connected to the port A1 via a check valve V₂ and thus to the dirty side 7 of the filter element F1. The spring-loaded check valve V₂ opens from port A2 to port A1 at a low opening pressure, for example at approx. 0.03 bar. The leak-oil line 5 is connected to the dirty side of a second filter element F2 serving as leak-oil-protection filter, which is connected to the tank side 3 on its clean side, via a further spring-loaded check valve V₄. The check valve V₄ is set, for example as a pressure relief valve on the leak-oil line 5, to an opening pressure of 0.5 bar, to limit the pressure of the leak-oil line 5 to this leak-oil pressure. In operation, the check valves V₂ and V₄ form a control device, configuring the filtration of incurred leak-oil in such a way that in operating phases, in which the working pressure at the port A1 is lower than the leak-oil pressure at port A2, the leak-oil is microfiltered via the filter element F1 via the open check valve V₂. When the working pressure at the port A1 increases due to large return quantities, due to high viscosities or due to contamination of the filter element F1, the valve V₂ closes, and at a leak-oil pressure at port A2 exceeding the opening pressure of the other check valve V₄, in this case 0.5 bar, the leak-oil from the leak-oil line 5 is filtered through the second filter element F2, which has, as a protective filter, a coarser grade of filtration, for example 40 μm, designed to prevent any detrimental leak-oil pressure from building up in the leak-oil line 5.

The working connection A1, and thus the microfiltering filter element F1, is protected by a bypass, which is formed by a further check valve V₃ between the ports A1 and A2, which may be set to an opening pressure of 4 bar, for instance. When this check valve V₃ is opened, a corresponding bypass volume is filtered via the opening check valve V₄ and the coarser protection filter F2. The protection filter F2 is in turn protected by an additional bypass 9, emanating from the leak-oil port A2 and running via a strainer F3, having a grade of filtration of 500 μm, for instance, as last-chance filter, via a spring-loaded check valve V₅ set to an opening pressure of about 2.0 bar, to the clean side 1 and thus to the tank side 3.

The circuit diagram of FIG. 2 corresponds to that of FIG. 1, except that on the clean side 1 a supply port B for supplying hydraulic components, for example a feed pump of downstream hydrostats, with a filtered working fluid, is provided.

In the circuit example of FIG. 3, a replenishment means 11 is provided as a development of the circuit of FIG. 2, in order to preclude any reduced supply at the supply port B. The replenishment means 11 has an unloaded check valve V₇, connecting the tank side 3 to the supply port B in case of negative pressure at the supply port B, as well as a spring-loaded check valve V₆, which, as a counterbalance valve, pre-loads the clean side 1 and thus the supply port B to a supply pressure of 0.5 bar.

The circuit diagram of FIG. 4 corresponds to that of FIG. 3, except that, between the suction valve forming the check valve V₇ and the tank T, a suction filter F4 has been inserted, with a filter grade of 100 μm, for instance. In addition, an additional leak-oil port A3 in parallel to the first leak-oil port A2 is provided for supplying leak-oil from a further closed hydraulic circuit.

The diagram of FIG. 5 corresponds to the circuit example of FIG. 3, except that the drain port A2 is not directly connected to the leak-oil line 5 leading to the protection filter F2 via the check valve V₄, but rather, the resulting leak-oil flow is led through an external leak-oil cooling circuit 13 with a heat exchanger 15, wherein the heat exchanger 15 is protected by a spring-loaded check valve V₁ with an opening pressure of 1.0 bar.

FIGS. 6 and 7 illustrate an exemplary embodiment of the filter device according to the present invention in front view and in longitudinal section, the corresponding hydraulic circuit having been realized in accordance with the circuit diagram of FIG. 1. The exemplary embodiment shows a circular cylindrical filter housing 17, tightly sealed by a head piece 19, which in turn is closed by a screw cap 21. At the other, bottom end, the housing 17 forms a bottom part 23 with outlets 25, 27 to an associated tank. The filter housing 17 can accommodate an interchangeable filter element 29, as shown separately in FIG. 8, featuring the filter element F1 serving as microfilter and the coarser protective filter F2 in a coaxial series arrangement. The filter elements F1 and F2 are, as can best be seen in FIG. 10, connected mechanically as well as connected to carry fluids by means of a valve insert 31 inserted between them. An end cap at the lower end of the protective filter F2 and an end cap at the upper end of the microfilter F1 are denoted by 33 and 35, respectively. A bypass valve device 37 is connected to the upper end of the upper end cap 35. The filter cartridge 29 of FIG. 8 is used both in the exemplary embodiment of FIGS. 6, 7 and 9 to 11 as well as in another exemplary embodiment shown in FIGS. 12 to 15, which will be discussed later.

As FIG. 6 shows, the working port A1, receiving the return flow from an open hydraulic circuit (not shown), as well as disposed above, the terminals A2 and A3, connected in parallel for incurred leak-oil quantities, are disposed at the head piece 19. The fluid paths routed in the head piece 19, housing 17 and bottom part 23, emanating from the ports A1 and A2, A3, are best illustrated in FIGS. 9, 10 and 11. As shown, the outer side of the filter media 39 and 41 of the filter elements F1 and F2, respectively, is located in each case at a spacing to the inner wall of the housing 17. As a result, a first fluid chamber 43 is formed on the outside of the upper filter element F1 and a second fluid chamber 45 is formed on the outside of the lower filter element F2. The first fluid chamber 43 is connected to the working port A1 carrying the return flow (cf. FIG. 9), i.e. the first fluid chamber 43 forms the dirty side 7 in the filtration at the filter element F1. The first upper fluid chamber 43 and the second lower fluid chamber 45 are separated from each other by a sealing arrangement having a gasket 47, which, as FIG. 10 shows, seals the valve core 31 against the inner wall of the valve housing 17. The check valve V₂ and the check valve V₄ are located in the valve core 31, disposed diametrically to each other. As can be taken from FIGS. 8 and 10 in combination, the gasket 47 has a slanted course, such that the first upper fluid chamber 43 is extended towards the lower filter element F2 downwards, beyond the check valve V₂, while the lower, second fluid chamber 45 is extended upwards via the other check valve V₄. As can be seen in FIGS. 8 and 10, the inclined gasket 47 is held on the valve plate 31 in a peripheral annular gap, which is limited laterally by two projecting ribs enclosing the gasket 47. These projecting ribs form a support against the inner wall of the valve housing 17 and thus safeguard against transverse forces owing to different pressures in the adjacent chambers.

In both filter elements F1 and F2, the filter medium 39 and 41, respectively, surrounds a support body 49 and 51, respectively. These each have a fluid-permeable outer wall 53 and a closed inner wall 55 located at a spacing thereto, which, between them, form a space 57, receiving, as the clean side 1, the hydraulic fluid filtered by the second filter element F2. The closed inner wall 55 of the support bodies 49, 51 forms an inner tube, formed by the connection of the valve core 31 as a continuous fluid path from the head piece 19 to the bottom part 23. The fluid path in this inner tube is connected to the leak-oil port A2, A3 in the head piece 19 through an opening 59 (FIG. 9) in the upper end cap 35 of the filter element F1 via an internal passage in the bypass valve device 37 with fluid connection, thus forming a part of the leak-oil line 5, cf. FIGS. 1 to 5. If the leak-oil pressure exceeds 0.5 bar, the check valve V₄ located in the valve insert 31 opens by lifting its closing body 44 towards the second lower fluid chamber 45. In this manner, port A2 is connected to the outside of the second filter element F2 serving as a protective filter, i.e. its dirty side. The fluid reaches the chamber 57 via the filter medium 41 of the filter element F2, and thus in turn the clean side 1. The respective clean side chambers 57 inside the support bodies 49 and 51 of the two filter elements F1, F2 are connected to each other by a flow around the valve insert 31, thus jointly forming the clean side 1 leading to the bottom part 23. In accordance with FIG. 10, the valve insert 31, as the valve housing of the check valves V₂ and V₄, widens a sleeve body 32 having a central passage 34, which continues the inner tube formed by the inner walls 55 of the support bodies 49, 51. On one side the valve insert 31 forms a holder for the adjacent end cap 36 of the upper filter element F1 and on the other side an end cap 38 enclosing the filter medium 41 of the lower filter element F2. As FIG. 11 shows most clearly, the clean side chamber 57 of the lower support body 51 is open towards the tank via the outlets 25 in the bottom part 23.

In the head piece 19, the spring-loaded check valve V₃ of the bypass valve assembly 37 can open a passage from the working port A1 to the opening 59 at the end cap 35 of the filter element F1. This occurs when an increase in pressure at the port A1 lifts a sleeve-like closing body 61 off from the edge of the opening 59 of the end cap 35 against the force of a valve spring 63 set to an opening pressure of 4 bar, cf. FIG. 9.

An additional bypass, designated 5 to 9 in FIG. 1 is formed at the lower end of the inner tube formed by the inner wall 55 of the support bodies 49, 51, by a spring-loaded closing body 65 of the check valve V₅ which, has been set to an opening pressure of 1.5 bar. The closing body 65 runs in a pipe section 75 forming the floor-side outlet 27 in the bottom part 23. The filter element F3 is located at the end closable by the closing body 65 of the tube formed by the inner wall of the support body 51, as a last-chance filter in the form of a strainer.

In the further exemplary embodiment of FIGS. 12 to 15, the circuit variant of FIG. 3 has been implemented. As FIGS. 12 and 13 show, the supply port B is located at the head piece 19 of the filter housing 17 in addition to the ports A1, A2 and A3, halfway between port A1 and ports A2 and A3 offset from these by 90°. In this exemplary embodiment, the central portion of the filter housing 17 is designed identically to the first exemplary embodiment, therefore corresponding to the illustration of FIG. 10. The head piece 19 is, however, designed differently and has a partition wall 71, between the inner wall of the head piece 19 and a hollow cylindrical extension 67 of the end cap 35, which surrounds the valve housing 69 of the bypass valve device 37 at a distance, separating the chamber in the head piece 19, to which port A1 leads, fluid-tight from the chamber, to which the ports A2, A3 lead, forming a central chamber 73, leading to the supply port B. As indicated by flow arrows 72, a fluid path runs from the clean side chamber 57 of the support body 49 of the filter element F1 via passages 74 in the end cap 35 to the chamber 73 via fluid guides in the neck 67 of the end cap 35, the channels 76 being bypassed. The latter form a fluid path from the opening 59 of the end cap 35 to the chamber of port A1, when the bypass valve device 67 responds to a lifting of the closing body 61.

In this exemplary embodiment, the bottom part 23 is also a modification of the previous example, cf. FIG. 15. The pipe 75 elongating the inner tube of the filter element F2, forming the tank-side outlet 27, surrounding the check valve V₅, forms, in conjunction with a coaxial cylindrical part 77 surrounding the tube 75 at a spacing thereto, a spring housing for the closing spring of the check valve V₆ forming a counterbalance valve. Its closing body 79, which is guided for lateral displacement in the spring housing, forms a seal of the clean side chamber 57 of the filter element F2 in the closed position, wherein passages 80 are formed in the closing body 79, however. The bottom of the cylindrical part 77 has, in addition to the penetration for the pipe 75, passages 81 as suction openings of the replenishment means 11, which, in conjunction with the associated closing bodies 82 form an unloaded check valve V₇ as a suction valve. The clean side of suction filter device 85 with the clean side of its filter element F4 is connected to the bottom of the cylindrical part 77 having the passages 81.

During operation of the filter device, the bottom part 23 being immersed in a tank, such that the suction openings forming passageways are immersed, the suction valve V₇ is normally closed, as there is no need for suction, by applying pressure via the passages 80. The closing spring 78 acts on the closing body 79 of the counterbalance valve V₆ in the closed position, resulting in the connection between the clean side chamber 57, i.e. the clean side 1, to the tank side via the outlets 25, being locked. Once the desired prestressing pressure, i.e. the supply system pressure for the supply port B, in this case by 0.5 bar, has been reached, the counterbalance valve V₆ opens, resulting in the closing body 79 opening the connection between the clean side chamber 57 and the outlets 25. If the pressure in the clean side chamber 57 is too low and the counterbalance valve V₆ is thus closed, the closure body 82 of the suction valve V₇ is lifted, resulting in a suction flow from the filter element F4 via the spring housing and the passages 80 into the closure member 79 of the counterbalance valve V₆. Under normal operating conditions where there is no need for suction, the operation of the exemplary embodiment corresponds to that of the example described above. 

1. A filter device having at least one first port (A1) at a hydraulic operating circuit, which has been assigned a filter element (F1), and having at least one additional port (A2), via which a leak-oil flow of hydraulic components can be conducted, characterized in that a control device (V₂) is provided by means of which the leakage oil from the additional port (A2) can be fed to the filter element (F1), if a pre-determined working pressure of the user circuit at the first port (A1) is not reached.
 2. The filter device according to claim 1, characterized in that the control device has a valve assembly having at least one check valve (V₂), which connects the additional port (A2) to the first port (A1), if the leak-oil pressure at the second port (A2) exceeds the working pressure at the first port (A1).
 3. Filter device according to claim 1, characterized in that the control device has a second check valve (V₄) connected to the additional port (A2), which opens when a desired range of the leakage oil pressure is exceeded and connects the additional port (A2) to a branch line leading to a return side or tank side (T).
 4. The filter device according to claim 1, characterized in that a second filter element (F2) is provided in the branch line leading from second check valve (V₄) to the return or tank side (T), having a filter grade which is coarser than that of the first filter element (F1) disposed between the first port (A1) and the return or tank side (T).
 5. The filter device according to claim 1, characterized in that the control unit has a third check valve (V₃), connecting the first port (A1) to the additional port (A2) as soon as a pre-determined maximum value of the working pressure at the first port (A1) is exceeded, thereby bypassing the first filter element (F1) via a second filter element (F2).
 6. The filter device according to claim 1, characterized in that an additional bypass, bypassing the second filter element (F2) leading to the return of tank side (T) provides a fourth check valve (V₅), which opens if pressure applied to the second port (A2).
 7. The filter device according to claim 1, characterized in that a strainer (F3) is located in the further bypass.
 8. The filter device according to claim 1, characterized in that a supply port (8) is connected to the clean side (1) of at least the first (F1) and/or second filter element (F2) for feeding filtered hydraulic fluid to hydrostatic components such as feed pumps.
 9. The filter device according to claim 1, characterized in that the clean side (1) of the first (F) and/or second filter element (F2), and thus the supply port (8), is connected to a tank (T) via a replenishment means, which has a fifth check valve (V₇) as a suction valve and a sixth check valve (V₆) for a desired pre-loading of the hydraulic fluid present at the supply port (8).
 10. The filter device according to claim 1, characterized in that the replenishment means (11) has a suction filter (F4), filtering the corresponding replenishment volumes, connected in series with the fifth check valve (V₇).
 11. The filter device according to claim 1, characterized in that it has a filter housing (17) defining a longitudinal axis, which can accommodate a first filter element (F1) serving as a microfilter and a second filter element (F2) in disposed sequentially, and interconnected with one another.
 12. The filter device according to claim 1, characterized in that the first filter element (F1) is adjacent to a removable head piece (19) and the second filter element (F2) is adjacent to a bottom part (23) of the housing (17).
 13. The filter device according to claim 1, characterized in that, in the housing (17), between its inner side and the outer side of the filter medium (39, 41), fluid chambers (43, 45) are formed between the first (F1) and the second filter element (F2), separated by a sealing arrangement (47), which is provided at the transition area of the filter elements (F1, F2), into an upper fluid chamber (43) assigned to the first filter element (F1) and a lower fluid chamber (45) assigned to the second filter element (F2).
 14. The filter device according to claim 1, characterized in that the check valves (V₂ and V₄) of the control device are disposed at the transition area of the filter elements (F1, F2).
 15. The filter device according to claim 1, characterized in that the external filter medium (39, 41) of the filter elements (F1, F2) surrounds a support body (49, 51) forming a chamber (57) between its fluid-permeable outer wall (53) and a closed inner wall (55), for both filter elements (F1, F2), which, as the clean side (1), receives the filtered hydraulic fluid.
 16. The filter device according to claim 1, characterized in that the closed inner wall (55) of the support bodies (49, 51) forms an inner tube as a continuous fluid path from the head piece (19) to the bottom part (23).
 17. Filter device according to claim 1, characterized in that the ports (A1 and A2), as well as fluid guides connecting the first port (A1) to the upper fluid chamber (43) and the second port (A2) to the fluid path in the inner tube formed by the inner wall (55) of the supporting bodies (49,51), are provided at the head piece (19).
 18. The filter device according to claim 1, characterized in that the third check valve (V₃) forming the pressure-operated bypass (37) of the first filter element (F1) is located between the ports (A1) and (A2) in the head piece (19).
 19. The filter device according to claim 1, characterized in that the clean-side chamber (57) inside the supporting bodies (49, 51) is connected to the return and tank side (T) via fluid paths (25,27) in the bottom part.
 20. The filter device according to claim 1, characterized in that the fourth check valve (V₅) in the bottom part (23), associated with the additional bypass, is disposed in such a manner that it normally closes the inner tube in the support bodies or opens an outlet of the inner tube leading to the return or tank side (T) actuated by pressure.
 21. The filter device according to claim 1, characterized in that the screen (F3) associated with the additional bypass is disposed upstream of the fourth check valve (V₅) at the end of the inner tube.
 22. The filter device according to claim 1, characterized in that the suction valve of the replenishment means (11), in the form of the fifth check valve (V₇), is provided at the outlets (81) of the bottom part (23), running from the clean side (1, 57) to the return or tank side (T).
 23. The filter device according to claim 1, characterized in that an additional outlet (25) leading to the tank (T) is provided in the bottom part (23) and that the sixth check valve (V₆), as a counterbalance valve, connects the clean side (1, 57) to the outlet (25) of the bottom part (23) when the desired amount of pre-loading has been reached.
 24. The filter device according to claim 1, characterized in that the replenishment means (11) has a suction filter (F4) between the outlets (81) of the bottom part (23) and the tank (T), which can be closed by the suction valve (V₇).
 25. The filter device according to claim 1, characterized in that the supply port (8) is disposed on the head piece (19) and can be connected to openings (74) via fluid guides (72), which form the access to the clean chamber (57) in the support body (51) in an assigned upper end cap (35) of the first filter element (F1).
 26. A filter element (29) for use in a filter device according to claim
 1. 